1. Field of the Invention
The present invention relates to a circulator that is applied to a radio frequency (RF) transceiver system and a radio frequency identification (RFID) reader having the circulator, and more particularly, to a digitally controlled circulator that can be mounted in a small portable terminal such as a cellular phone and an RFID reader having the circulator.
2. Discussion of Related Art
RFID is a technology that reads information from a tag or writes information to a tag using an RF and is used to identify, trace and manage goods, animals, people and so forth to which a tag is attached. An RFID system based on the RFID technology comprises a tag or a transponder that has unique identification information and is attached to goods, people or so forth; a reader that reads the identification information from the tag or writes information to the tag; an object database; a network; and so forth.
Lately, an ultra high frequency (UHF) band RFID system is used to manage the distribution of goods. In addition, since an electronic product code (EPC) global generation-2 standard (refer to EPC Gen-2 standardization documents) and an international standardization organization (ISO) international standard are established as the standards of an air-interface for RFID, the UHF-band RFID system is expected to be extensively used, the EPC global generation-2 standard and ISO international standard aiming at assigning consistent unique codes to all goods mainly through an Auto-ID center.
FIGS. 1a and 1b are block diagrams illustrating a RFID reader used in conventional RFID system.
Referring to FIG. 1a, a signal carrying tag information is sent from a processor 100, which includes a microprocessor 101 and a memory 102, to a modem 104 through a bus line 103. The RF signal is sent to a transmitter 105 through the modem 104 and sent to a tag through an antenna 110. In addition, tag information that is received through the antenna 110 is sent to a receiver 106, and is sent to the processor 100 through the bus line 103 by the modem 104.
When the RF signal is sent from the transmitter 105 to the tag, a circulator 120 segregates the receiver 106 from the transmitter 105 to protect the receiver 106, the circulator 120 being disposed among the transmitter 105, the receiver 106, and the antenna 110.
Referring to FIG. 1b, the RFID reader has a structure in which an antenna 140 for sending an RF signal is separated from another antenna 141 for receiving an RF signal. A signal carrying tag information is sent from a processor 130 to a modem 134 through a bus line 133, the processor 130 consisting of a microprocessor 131 and a memory 132. The RF signal is sent to a transmitter 135 through the modem 134 and sent to a tag through the antenna 140. In addition, the tag information that is received through the antenna 141 is sent to a receiver 136, and is sent to the processor 130 through the bus line 103 by the modem 134.
The ferrite circulator 120 that operates as described above has excellent electrical performance. However, since the circulator 120 is made up of a great-sized magnetic material which cannot be fabricated by a general integrated circuit (IC) semiconductor process, it is not suitable for small, highly integrated, and inexpensive readers.
In addition, when the structure having the two antennas 140 and 141 as described above is applied to the system using a UHF band, dimensions occupied by the antennas increase, so that it is difficult to apply the structure to small devices such as cellular phones.
Therefore, in order to solve the problems, provided are an active circulator comprising transistors and a passive circulator based on a Wilkinson power splitter.
FIG. 2a shows an example of a conventional active circulator comprising transistors. In the active circulator, an antenna is connected to a first terminal P1 of a power splitter, and a signal that reflects from a tag and is received through the antenna is equally distributed to a second terminal P2 connected with a transmitter and a third terminal P3 connected with a receiver. Here, matching networks 210 and 220 are respectively connected with the second terminal P2 and the third terminal P3, and transistors T1 and T2 are respectively connected between the matching network 210 and the first terminal P1 and between the matching network 220 and the first terminal P1. The drain of the transistor T1 is connected with the first terminal P1 through a microstrip transmission line 201, the source of the transistor T1 is connected with the gate of the transistor T2 through a microstrip transmission line 202, and the gate of the transistor T2 is connected to a ground terminal through a microstrip transmission line 203.
FIG. 2b shows an example of a conventional passive circulator based on a Wilkinson power splitter. The passive circulator is a quasi-circulator including microstrip transmission lines on the basis of the principle of the Wilkinson power splitter.
An antenna is connected to a first terminal P1 of a power splitter, and a signal that reflects from a tag and is received through the antenna is equally distributed to a second terminal P2 connected to a transmitter and a third terminal P3 connected to a receiver and thus is transmitted. Here, a microstrip transmission line 230 is connected between the first terminal P1 and the second terminal P2, and a microstrip transmission line 231 is connected between the first terminal P1 and the third terminal P3.
The circulator that comprises only electronic devices as described above is easily integrated into a monolithic microwave integrated circuit (MMIC) or a radio frequency integrated circuit (RFIC). However, in the circulator comprising the transistors, the sensitivity of the receiver deteriorates due to noise generated at the transistors T1 and T2, and the transmitter barely processes a high output signal of more than 20 dBm without distortion due to a nonlinearity characteristic. Generally, when an active circulator is used, power can be transmitted without loss. However, it is known that the active circulator has worse characteristics than a passive circulator due to a very high noise figure (NF) (See “IEEE Trans. Microwave Theory and Tech.,” G. Carchon and B. Nauwelaers, Vol. 48, No. 2, pp. 316-319, 2000).
Meanwhile, in the passive circulator based on the Wilkinson power splitter, electrical characteristics can be fixed by equivalent inductance and capacitance. The equal distribution of the signal received through the antenna to the transmitter and the receiver, however, causes an insertion loss of 3 dB. This insertion loss increases the NF, deteriorating receive sensitivity, and increases a bit error rate (BER). Therefore, the passive circulator based on the Wilkinson power splitter is difficult to apply to an RFID system requiring a very low BER.